PHYLUM ECHINODERMATA 191 



at the oral surface (Fig. 132) reveals a mouth centrally situated 

 in the membranous peristome, and five grooves (ambulacral), one 

 in each arm, from which two or four rows of tube-feet extend 

 (Fig. 133, 17). 



The Skeleton. The skeleton is made up of calcareous plates 

 or ossicles bound together by fibers of connective tissue (Fig. 

 133, 9, n, 12). The ossicles are regularly arranged about the 



FIG. 132. A, the starfish, Asterias rubens, seen from the oral surface. 

 B, an adambulacral spine, showing three straight pedicellarue. C, a tube- 

 foot expanded and contracted. (From the Cambridge Natural History.) 



mouth and in the ambulacral grooves and often along the sides 

 of the arms, but are more or less scattered elsewhere. The am- 

 bulacral and adambulacral ossicles (Fig. 133, 11, 12] have muscu- 

 lar attachments and are so situated that when the animal is 

 disturbed they are able to close the groove and thus protect the 

 tube-feet. The spines of the starfish (Fig. 131; Fig. 133, 8) are 



